At least nine people were taken to hospitals amid the shooting that spanned several hours. At least four police officers and possibly a fifth were wounded, police said.
The gunman also planted unspecified "items" outside the building and carried bags inside, police Lt. Catherine Buckley said. Authorities planned to investigate the items once the scene was stabilized "to make sure they're not any kind of improvised device," she said.
Nearly five hours after the first report of gunfire, authorities said the shooter was still not in custody and that an unknown number people were evacuated — some wrapped in blankets in the blowing snow — to a nearby Veterans Administration clinic.
Three officers were injured while responding to the initial report of shots fired at the clinic before noon, authorities said. More than two hours later, the gunman shot and injured a fourth officer in another exchange with police inside the clinic, Buckley said. She said later a fifth officer might be hurt.
Officers have not had any voice contact with the shooter and do not know whether anybody was being held hostage. Authorities said they don't know the motive of the gunman or whether the shooter had any connection to Planned Parenthood.
"We don't have any information on this individual's mentality, or his ideas or ideology," Buckley said.
Planned Parenthood released a statement that said it did not know the full circumstances or motives behind the attack, or whether the organization was the target.
The shots sent people inside the clinic racing for cover. Jennifer Motolinia hid behind a table inside the clinic and called her brother, Joan, who said he heard multiple gunshots in the background.
"She was telling me to take care of her babies because she could get killed," Joan Motolinia said of his sister, the mother of three.
He rushed to the clinic but was frustrated because a police barricade kept him from getting close.
"People were shooting for sure. I heard someone shooting. There was a lot of gunfire. She was calm, she was trying to hide from those people," he said.
Police cordoned off the clinic, nearby medical offices and a shopping center. Authorities ordered everyone in the area to take shelter where they were.
Denise Speller, manager of a nearby hair salon, said she heard as many as 20 gunshots in less than five minutes.
She told The Gazette newspaper that she saw a police cruiser and two officers near a Chase Bank branch, not far from the Planned Parenthood facility.
One of the officers appeared to fall to the ground and the other officer knelt down to help and then tried to get the officer to safety behind the car, she said. Another officer told Speller to seek shelter inside the building.
"We're still pretty freaked out," Speller said by phone. "We can't stop shaking."
Ambulances and police vehicles lined up at a nearby intersection and police told people via Twitter to stay away from the shooting scene because it was not secure.
Shelley Satulla said she saw five or six people put on stretchers and placed in ambulances lined up next to King Soopers shopping center near the clinic.
Later in the afternoon, other people were able to walk out of the shopping center area accompanied by police officers toward a line of ambulances.
Mike Pelosi, who works at a deli at a nearby King Sooper grocery store, said he heard over the store's loudspeaker just before noon that nobody could leave the store.
Pelosi said customers and store employees were confused about what was going on but not panicked. He said a couple dozen customers were standing near the store entrance waiting for instructions.
The location of the shooting is less than 6 miles from the street where a man shot and killed two of three people before dying in a gun battle with police on Halloween day.
Authorities said 33-year-old Noah Jacob Harpham first shot a bicyclist near his home, then walked less than a mile and shot two women on the porch of a sobriety house.